Electrical power converters are devices for processing electrical power from one form, such as an AC or unregulated DC voltage, into another form, such as one or more regulated DC output voltages. One conventional type of electrical power converter that produces a regulated output voltage is a switching power supply, also commonly referred to as a switch mode power supply or a switched power supply.
Conventional switching power supplies commonly include a power transformer and one or more power switches for alternately coupling an unregulated DC or rectified AC voltage across a primary winding of the power transformer in a series of voltage pulses. These pulses are transformed into a series of voltage pulses across one or more secondary windings of the power transformer and then rectified and filtered to provide one or more output DC voltages. Power transformers conventionally include a ferrite or tape-wound core and at least one primary and one secondary winding. Power switches commonly are metal-oxide semiconductor field-effect transistors (MOSFETs or FETs), although other types of transistors (such as bipolar junction power transistors, BJTs) are sometimes used.
The output voltage or voltages of the power converter are commonly regulated by controlling the duration of the voltage pulses coupled to the primary winding of the power transformer, thereby controlling the duration of the voltage pulses produced across the secondary windings of the power transformer. The relative amount of each switching cycle that the power switches are on, allowing power to be coupled to the power transformer, is referred to as the "duty cycle" at which the power converter is operating. For light loads (i.e., loads drawing a relatively small amount of power), the output voltage may be maintained by coupling a relatively small amount of power across the power transformer. Thus, for light loads, the power converter is operated at a low duty cycle, resulting in narrow (i.e. short duration) voltage pulses being coupled to the power transformer. Conversely, for heavy loads (i.e., loads drawing a relatively large amount of power), the output voltage is maintained by operating the power converter at a high duty cycle, with correspondingly wide voltage pulses being coupled to the power transformer.
One common type of switching power supply is the flyback power converter. In a conventional flyback converter, a rectifier is coupled to the secondary winding of the power transformer such that current is prevented from flowing through the secondary winding when voltage is coupled across the primary winding. Thus, at the beginning of each switching cycle of a flyback power converter, the power switch turns on and couples a voltage across the primary winding such that current in the primary winding ramps up from zero, thereby storing energy in the form of magnetic energy in the power transformer. The period of time during which the power switch is on is referred to as the drive cycle or drive period. Turn off of the power switch conventionally takes place in response to the level of an output voltage and/or the mount of current flowing through the primary winding of the power transformer. After the switch is turned off the energy stored in the power transformer is released through the secondary winding and filtered to produce the desired output voltage. The period of time during which energy is released from the secondary winding is referred to as the flyback cycle or flyback period. After essentially all stored energy is released, the power switch is again turned on and the switching cycle repeats. In conventional flyback power supplies, the power switch often is maintained in its ON state during the drive cycle by a tertiary winding of the power transformer. For example, the power switch is commonly a MOSFET having its gate coupled to a tertiary winding of the power transformer such that a sufficient voltage induced across the auxiliary winding will maintain the MOSFET in the ON state during the drive cycle.
Power transformers do not lend themselves easily to miniaturization and, thus transformers can place minimum size and cost limitations on power supply design. For this reason, one consideration in the design of electrical power converters is to minimize the size of the power transformer. To that end, an inductor (or choke) may be placed in parallel with the secondary winding of the power transformer. This increases the energy storage capability of the circuit for a given transformer size. However, due to the different times necessary to reset the cores of the inductor and the transformer a DC bias is induced in the power transformer core. The maximum acceptable DC bias point is temperature dependent because the saturation level of the core decreases when temperature increases. If the transformer is allowed to become saturated, excessive current is coupled through the power switch, causing the switch to fail.
As an alternative to using an energy storage choke, in certain electrical power converters the power transformer core has an air gap in it such that there is no closed magnetic circuit entirely within ferromagnetic material. For example, in certain converters having a toroidal core transformer, a gap separates two ends of the core. An air gap allows greater current capability before the onset of magnetic saturation preventing the saturation induced current and voltage spikes that can destroy power transistors. However, because air gaps decrease the average inductance of transformers, the size of the air gaps must be minimized. Otherwise the core size (and cost) of the transformer would have to be increased to provide the required level of inductance. An additional limitation of this approach is that the necessary gap length for many applications is too small to be obtained by conventional machining or taping.
As is well known in order to minimize the size of a flyback power transformer it is also preferable to operate the power converter at a high power switching frequency so that electrical current being conducted through the power transformer windings is less likely to reach a level sufficient to cause saturation of the transformer core. Problems with this approach, existing even if the transformer core has a conventional air gap, are related to the safe operating area (SOA) of the power switching device. In a power MOSFET, for example, the SOA is defined by the maximum drain current, the maximum drain-to-source voltage, and the maximum power dissipation. At high line input or light output load current (and thus low duty cycle), the gate-drive pulse for the power switch is relatively narrow (i.e., short in duration). At high switching frequencies, the pulse is often 500 nanoseconds or less in duration. Power switches require a finite amount of time to turn on fully (e.g., 100 nanoseconds). Thus, the power switch often is not completely turned for a high percentage of the drive cycle. A substantial amount of current nevertheless passes through the power switch during each drive cycle while the switch is not completely turned on. Such circumstances result in the power switch being required to dissipate a very large amount of power in the form of heat. In such cases, the maximum power dissipation level of the power switch is commonly exceeded and the switch is burned out.
At narrow pulses a phenomenon known as thermal runaway can also occur in some types of power switches (most commonly BJTs) even if the maximum power dissipation limits would not otherwise be exceeded. In such cases, the temperature of a switch rises due to power dissipation, and the higher temperature causes the switch to conduct more current, resulting in higher temperatures. This cycle of increasing temperatures and currents continues until excessively high temperatures are reached and the switch destroys itself. A similar effect can occur in small portions of the junction areas of transistors, causing a local thermal runaway known as "second breakdown." The phenomena of thermal runaway and second breakdown further limit the SOA of some switches.
Accordingly, there is a need for reducing the amount of power that must be dissipated in the power switch of a flyback electrical power converter operating at low duty cycles.